implement CAutoFile via std::fstream

Not intended to get merged, I just need a pulltester-check ;-), thanks!

- implement CAutoFile via std::fstream and use it in the code
- unify log/error messages for serializing/deserializing exceptions
- add debug and benchmark messages for writing/reading block/undo files
- remove boost::path member from CAddrDB class
- add new helper functions GetBlockFile() and GetUndoFile()